A key problem of current DF systems is their large size, weight, power consumption, and setup time, when they must operate at low frequencies, such as a less than a few MHz. For many years, the government has advertised requests for proposals to solve this problem. Many journal articles from radio operators to government researchers to university professors, have been written attempting to solve this problem. What is desired is an RF emitter sensing device that operates at low frequencies and particularly at less than a few MHz, that is also small enough to be handheld or man wearable (e.g. contained within a backpack or in an operators clothing), or small and light enough to fly on a miniature unmanned aerial vehicle (UAV). The disclosed RF emitter sensing device is a solution to this long-standing problem. The disclosed RF emitter sensing system includes (a) antennas that are unique in that they are extremely wide bandwidth and their directivity (i.e. front-to-back ratio) improves as the frequency goes down, allowing the DF system to operate to arbitrarily low frequency regardless of how small it is, and (b) signal processing methods to enhance its sensitivity and accuracy to help mitigate the fact that the energy collection area of the miniature antennas is small. The small high directivity antennas and the signal processing methods, taken together, create the long asked for DF system.
The angle-of-arrival (AoA) or direction-of-arrival (DoA) of a signal of interest (SoI), along with range and polarization, can be expressed in a spherical coordinate system, such as pictured in FIG. 15. Standard geometric rotation and translation calculations can be used to change on object's pose (position and orientation) within a coordinate system or to convert a pose between coordinate systems. In this document we will use the term AoA to mean either a single angle, such as azimuth, or the combination of angles, such as azimuth and elevation, in a defined coordinate system.
For example, in an earth-centric 2D planer coordinate system with the plane parallel to a point on the earth's surface, AoA typically means an azimuth angle, or in other words, a compass direction. The plane could be pictured as the x-y plane in FIG. 15. Azimuth is sometimes measured as a counter-clockwise angle from east where 0 degrees means due east, 90 degrees means due north, and the angular range covers from 0 to 360 degrees. With this azimuth angle definition, in FIG. 15, there would be no z-axis since it is a 2D coordinate system, the x-axis would aim due east, the y-axis would aim due north, and φ would be the azimuth angle.
For another example, in an earth-centric 3D coordinate system, AoA typically means a combination of angles, an azimuth-angle and an elevation-angle. The elevation-angle is typically understood to be an angle covering −90 to 90 degrees relative to a plane parallel to the surface of the earth, where 90 degrees means straight up from the earth's surface toward outer space, −90 degrees means straight down toward the center of the earth, and 0-degrees means parallel to the earth's surface. Given this elevation angle definition, in FIG. 15, 90-θ degrees would be the elevation angle.
Depending on the application, the desired output of the RF emitter sensing system may be either 2D or 3D. Typically, different applications have different lists of desired outputs that also include items such as the SoI's polarization, frequency, magnitude, duty-cycle, peak-to-average ratio, repetition rate, modulation type, event time and the confidence level of these estimates. A confidence level is a statement such as 95% of the estimates will have an error of less than a given amount like, for example, 1 degree, or 10 Hz, or 2 dB, etc.